What uses are there for a marijuana plant?

If you are growing it yourself then you already know.

If you find a single plant you may pull it out by the roots and compost it.

If you find a large patch and it seems to be well tended, you should carefully leave the area as quickly as possible and report the location to your nearest police department.

Illegal growers are very protective of their crops and could do you harm if they find you near it.